Titanium and titanium alloys find extensive use as the anti-corrosion material used in chemical plants or like apparatus. It is used in severely corrosive environment or of component parts of such apparatus. However, where non-oxidized solutions such as hydrochloric acid solutions are handled, active dissolution of titanium occurs. Also, where chloride solutions at high temperatures are handled, the problem of abnormal corrosion of inner intersticial parts of apparatus or crevice corrosion has not yet been solved.
Chemical apparatus has crevice in various parts, typical examples of which are flange points of liquid ducts at the gaskets. It will be readily understood that chemicals within the apparatus intrude into such crevice portions. Although the titanium material inhereintly has excellent corrosion resisting properties, its superficial portions cannot be perfectly immune to corrosion. When a reducing reaction proceeds in superficial portions and crevice portions within the apparatus, the concentration of the dissolved oxygen in such portions is reduced. While the superficial portions within the apparatus are replenished with dissolved oxygen from other portions (for instance central portion within the apparatus), replenishment of the dissolved oxygen in the crevices can hardly be expected. Consequently, oxygen concentration is reduced in effect only in the crevice portion, giving rise to the formation of a so-called oxygen concentration cell with the crevice portion acting as anode inducing an electrochemical reaction represented as EQU Ti + 2H.sub.2 O .fwdarw. TiO.sub.2 + 4H.sup.+ + 4e.sup.-
Thus, there results in an increase in hydrogen ion concentration and in the concentration of chloride ions in the crevice portion which cause the reaction as represented by EQU Ti .fwdarw. Ti.sup.+++ + 3e.sup.- ,
to occur, thus leading to abnormal corrosion of the crevice surface. In this case, although the majority of the inner surfaces of the apparatus are free from corrosion, leakage of process material from crevice portions, for instance pipe joints, is liable to result. This drastically lowers the safety of the entire chemical apparatus.
Known methods of preventing such corrosion include:
(1) using a titanium alloy containing 0.1 to 0.2 percent of palladium: and
(2) in which a platinum group element is deposited on the surface of the titanium material with or without subsequent diffusing treatment. However, the first method is economically disadvantageous because the .pi.-Pd alloy uses a great quantity of expensive palladium, while the second method dictates complications of the manufacture of the apparatus, as well as calling for the consumption of a great quantity of the platinum group element.
The titanium material also has another drawback in that it tends to become embrittled hydrogen under high-temperature, high-pressure conditions. Under such circumstances the index of hydrogen absorption is high since generation of hydrogen due to corrosion reactions is highly possible, and this problem has recently become important. This tendency of the titanium material to become fragile due to hydrogen absorption is said to be attributable to the facts that hydrogen atoms immediately after generation in the cathode region are very apt to react with titanium and that the resultant hydride of titanium is very fragile. In order to prevent this absorption of hydrogen into titanium, it has been proposed to:
(1) deposite a metal which is potentially nobler than titanium, for instance platinum and palladium, on titanium: or
(2) subject the titanium material to anodizing or chemical oxidation treatment with chromic acid solution.
The former method is economically not practical for the same reasons as mentioned earlier, and also the range of its application is limited. In the second method, the oxide layer produced is reduced in a short period of time, so that it is impossible to expect a great effect of preventing hydrogen absorption.